In general, boilers are machines for mixing and burning air and fuel. In these burners, combustion efficiency is determined depending on how accurately the mixing ratio of the air and the fuel is controlled.
In the boilers, an air proportional control system, or a current proportional control system is applied in order to control the air-fuel ratio. An accurate quantity of fuel is supplied in proportion to the pressure of air introduced by controlling the revolutions per minute (rpm) of a fan through the mode, and thereby the boilers can be increased in combustion efficiency and inhibit harmful gases from being discharged to the maximum extent. Here, the air proportional control system is adapted to appropriately maintain the air-fuel ratio by calculating the volume of supplied air for combustion based on a required calorie in terms of the rpm of a fan, automatically opening/closing an air proportional valve by the pressure of air introduced in proportion to the calculated rpm of the fan, and supplying a proper quantity of fuel. The current pro-portional control system is adapted to appropriately maintain the air-fuel ratio by calculating the volume of supplied air for combustion based on a required calorie in terms of the rpm of a fan, and controlling the current value of a current proportional valve for supplying fuel in correspondence to the rpm of the fan.
Further, there has recently appeared a boiler having a air pressure sensor allowed to measure the pressure of introduced air using differential pressure. As shown in FIG. 1, the boiler is comprised of a fan 10 sucking in air for combustion, a air pressure sensor 20 measuring the pressure of air sucked in by the fan 10, and a controller 30 receiving data on the air pressure measured by the air pressure sensor 20, controlling a quantity of fuel supplied through a fuel supply pipe 40 according to the input air pressure, adjusting the rpm of the fan 10 when the pressure of air sucked in is varied depending on the quantity of supplied fuel, and controlling the fan so as to allow a proper volume of air to be maintained based on the quantity of supplied fuel through the voltage measured by the air pressure sensor.
Although not shown, a venturi tube is installed on an air passage in the fan 10 so as to generate differential pressure through variation in cross section. The air pressure sensor 20 is connected to the venturi tube, thereby measuring the air pressure using the differential pressure generated at the venturi tube.
In this manner, the air pressure sensor 20, which measures the air pressure using the differential pressure, is composed of a diaphragm 21 moving up and down depending on variation of the air pressure, an elastic member 22, such as a spring, applying resilient force to the diaphragm 21, a reflecting member 23 going upward/downward according to up-down movement of the diaphragm 21, and a sensing member 24 sensing a difference in the movement of the reflecting member 23 to measure pressure.
Here, the sensing member 24 includes a light emitting element and a light receiving element. When the light emitting element emits light, the light is reflected on the reflecting member 23, and then the reflected light is received by the light receiving element. The sensing is based on principle that, as current is varied in proportion to a quantity of light received by the light receiving element, a value of measured voltage is varied.
The air pressure sensor 20 taking this construction and operation is disclosed in Korean Utility Model Registration No. 0353005, filed by the present applicant, the entire contents of which are incorporated herein by reference.
According to the conventional boiler having the above-mentioned air pressure sensor, it is possible not only to increase the combustion efficiency by the aid of the proper air-fuel ratio, but also have precise control so as to have the air-fuel ratio according to rated output required for the boiler. However, the conventional boiler has disadvantages in that an initial reference voltage value is varied with the arrival of hot/cold weather, that sensor properties are varied when used for a long time, and that an initial reference value is varied by dust or foreign materials on the reflecting member, variation of the resilient force of the elastic member, and so on.
In other words, the conventional boiler fails to eliminate various noise factors, such as property deviation of the air pressure sensor, deviation caused by resilient property of the spring or the elastic member, deviation caused by colors of the reflecting member, deviation caused by the rubber property of the diaphragm, and so on, so that it cannot control the optimal combustion efficiency or optimally maximized air-fuel ratio.
The description will be made in more detail with reference to FIG. 3. FIG. 3 is a graph plotting the relationship between voltage and pressure when a boiler operates. When the fan is driven, ordinary voltage of the fan should plot a standard curve graph having reference start voltage of 0.3 V and reference peak voltage of 3.2 V. However, due to the above-mentioned noise factors, the ordinary voltage of the fan plots an up-curve or down-curve graph on which both the start voltage and the peak voltage are equally raised or lowered on the whole, compared to the standard curve graph. Thereby, it is impossible to control the optimal combustion efficiency and the precise air-fuel ratio. Furthermore, the reliability of the products is lowered.